This invention relates to electric fail-safe systems and more particularly to a system for safeguarding against fluid amplifier and wiring failures in an aircraft servo control system.
It has become common particularly in the aircraft fly-by-wire systems to employ redundant devices for safequarding against failure of any one component of the system, and it is known for example to employ multiple valving arrangements to achieve such redundancy.
Thus, for example in U.S. Pat. No. 4,138,088 entitled Device for Controlling Hydraulic Motors, there is disclosed a control system for an aircraft actuator which utilizes four fluid amplifiers or electro-hydraulic servo valves acting in pairs and with the pairs of valves acting in tandem to cause movement of a valve spool, which in turn through associated valving structure causes a desired movement of an output device, typically a hydraulic actuator connected by suitable linkage to a control surface of the aircraft. The fluid amplifiers in such arrangement are controlled in separate channels of command information and any one of the fluid amplifiers is capable of effecting the desired output for the actuator element. This is accomplished by amplifiers acting in concert upon a common valve spool in a mechanical summing arrangement. In such arrangement two pairs of fluid amplifiers are mechanically interconnected at one end of the spool for the common actuation of the valve spool with loss of any one fluid amplifier being detected through angled movement of a pivoted connecting member. Such type of failure is monitored to create an electrical signal which is then utilized to energize a solenoid valve to hydraulically disengage the faulty fluid amplifier pair from the control system allowing the remaining pair of fluid amplifiers at the other end of the valve spool to provide the control function in an unhindered manner.
Fluid amplifier pairs are employed in such control system not only for mechanical redundancy but also so that duplicate channels of electrical fly-by-wire information may be delivered to the system from a remote command location. In such system failure of any one pair of amplifiers due to electrical fault is also accommodated by continued operation of the second redundant pair in a manner previously described. In still further variations of such systems and in order to further increase reliability of the electrical portion of the control, as this is usually considered more susceptible to failure than that of the mechanical elements, arrangements have been devised wherein four channels of command input information in redundant format are applied to all of the fluid amplifiers in a servo control system of the type described previously. In such system, each one of the fluid amplifiers includes four separate coil windings therein adapted for energization by the four input channels of information. Each of the channel coils of the fluid amplifiers is connected to the respective channel coil of other fluid amplifiers in a series circuit configuration to provide a common signal to all of the fluid amplifiers, sufficient to actuate same. Thus with four channels of input command information applied to all of the fluid amplifiers, reliability of the system is increased considerably in that separate routings of the signals may be made through the structure of the aircraft to accommodate damage to any one portion thereof while still maintaining full fluid control dynamic capability at the fluid actuator.
In achieving this increased reliability however, the system has become particularly susceptible to the type of failure in which all of the coils in any one particular fluid amplifier might become faulted through a common incident, creating an open circuit condition in all of the four channels of information, obviating the advantage of having the redundant channels. While an open circuit condition in any one channel or up to three of the channels may be accommodated by the redundancy of the remaining channels, the particular fault condition described which is common to all of the channels has not found a solution in the prior art.